In the operation of a conventional tandem strip mill, a metal slab or bar, heated to a suitable hot-rolling temperature, is introduced to the first of a series of roll stands and passes successively through the other stands, which reduce it in steps to strip form. In each stand the gap between rolls is smaller than in the preceding stand and the rolls are driven at a faster rate to handle the lengthening strip. Each stand is equipped with screws and screwdown motors for adjusting the relative position of the rolls and the size of gap between rolls. When a strip is actually between the rolls of a stand, the roll housings stretch. Hence during a rolling operation the actual gap is the algebraic sum of the setting obtained by adjustment of the screws and the stretch in the roll housings.
In setting up the mill, the positions of the rolls are adjusted beforehand to provide gaps which are smaller than the desired gap to allow for stretch in the housings when the strip is between the rolls. As the housings stretch, the gap becomes approximately correct for rolling strip of the desired gauge. In roll stands other than the first the rolls may be set "below face"; that is, the rolls are in contact and actually stretching the housings even though no strip is present.
Conventionally an X-ray gauge is used to scan the strip as it leaves the last stand. If the strip is off-gauge, the X-ray gauge generates a signal which automatically operates the screwdown motors of some or all the stands to correct the gauge error. Adjustments thus obtained would maintain the rolls at proper setting only if there were no variations in the physical characteristics of the strip. In practice a strip becomes progressively cooler, and hence harder, from its leading end to its trailing end. This fact necessitates tightening the screws progressively throughout a rolling operation to maintain the gaps at the proper size. Apart from normal cooling, the strip has portions of lower temperature than normal as a result of contact of the original slab or bar with skids in the reheating furnace, or other heat absorbing objects. When such cooler portions are between the rolls of a stand, the magnitude of force tending to separate the rolls increases. Any change in the roll-separating force changes the stretch in the roll housings and, unless corrected, changes the roll gap and produces a gauge error in the strip.
To correct gap errors which would be caused by variations in the strip, it is known to equip the mill with automatic gauge control (AGC) apparatus, and there are numerous patents showing such apparatus. Essentially AGC apparatus includes load cells installed on some or all the stands to measure the roll-separating force, and electronic circuits and sometimes a digital computer connected to the load cells and to certain of the screwdown motors. As the strip becomes progressively harder along its length, or when a portion of the strip between the rolls has characteristics other than normal, the load cells generate signals which effect screw adjustments at one or more stands. Thus AGC apparatus maintains the roll gap at the adjusted stands at its desired constant size, as corrected by signals from the X-ray gauge, despite variations in the roll-separating force.
In one form of AGC apparatus used heretofore, the load cells of a first stand N are tied to the screwdown motors of the same stand. If the roll-separating force at this stand increases, the screwdown motors of this stand operate in a direction to tighten the screws at this stand. This leads to a problem that tightening the screws further increases the roll-separating force. Hence the screwdown motors must be stopped short of full correction to prevent their "running away". To obtain full correction, one or more following stands N + 1, N + 2, etc. operate as slave stands, whereby their screwdown motors operate in response to signals from the first or master stand N to effect the same or larger screw adjustments. Reference can be made to Wallace et al. U.S. Pat. No. 3,357,217 for a showing of an AGC apparatus which operates in this fashion.
Other earlier forms of AGC utilize a partial feed-forward principle. Load cells installed on one stand N, detect changes in the roll-separating force at this stand, produce signals which effect screw adjustments at this same stand, and transmit signals representative of such changes to following stands N + 1, N + 2, etc., where they may effect further screw adjustments. Transmission of the signals to following stands is delayed to allow for transport time of the strip between stands, but to the best of my knowledge the reaction time of the screws has not been taken into account. Such AGC apparatus are said to overcome certain problems encountered with the AGC apparatus of the master-slave type described above. Reference can be made to Coleman et al. U.S. Pat. No. 3,448,600, Masar U.S. Pat. No. 3,702,071, or Smith U.S. Pat. No. 3,709,008 for showings. Reference also can be made to Arimura et al. U.S. Pat. No. 3,677,045, Fox et al. U.S. Pat. No. 3,841,123, Peterson et al. U.S. Pat No. 3,848,443, or Fox U.S. Pat. No. 3,851,509 for other AGC showings.
Whenever gap-error signals generated at the first few stands are fed-forward to effect screw adjustments at a succeeding stand, the adjustment must be delayed until the portion of the strip for which an adjustment is needed arrives at the stand where the adjustment is to be made. Delay means used heretofore have been unduly complex and costly. The load cell on a roll stand generates analog voltage signals representative of changes in the roll-separating force from normal. Usually the analog signals have been converted to digital signals, and the digital signals have been delayed and converted back to analog signals to operate the screwdown motors. The AGC apparatus shown in the aforementioned Coleman et al. patent is an example.
In addition to adjusting the roll gap to control strip gauge, the tension in the strip may be adjusted to effect gauge control. Conventional tandem strip rolling mills usually include one or more loopers between roll stands. These loopers can be used to vary the tension in the strip and to assist in gauge control, since increasing the tension produces a thinner strip. This practice is undesirable since tensioning the strip not only reduces the gauge, but also reduces the width, which should be held constant.